Soil microbes are releasing nitrous acid into the atmosphere through a process scientists have only just begun to understand. This matters because current climate models treat soil emissions of reactive nitrogen as a minor afterthought, yet these emissions are projected to rise from 60 to as much as 130 teragrams of nitrogen per year by 2100 as global temperatures climb. The gap is critical: nitrous acid acts as a precursor for atmospheric radicals that shape air quality and climate, but the microbial mechanisms behind its production remain unknown. This project will test whether ammonia-oxidising bacteria actively release nitrous acid while archaea do not, and determine the molecular pathways involved. If successful, the work could reveal a previously unrecognised link between the nitrogen and carbon cycles—nitrous acid may drive abiotic photooxidation of soil organic matter, accelerating carbon loss from soils. Improved representation of these processes in Earth system models would sharpen climate change forecasts and inform sustainable land management. The research is fundamental science, but understanding this hidden microbial chemistry could ultimately reshape how we predict soil carbon storage and atmospheric chemistry in a warming world.
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Atmospheric reactive nitrogen oxides (NOy = NO + NO2 + HONO + …) are coupled to Earth’s nitrogen (N) cycle through interactions of anthropogenic activity, soil N, and soil microbial activity. The biogeochemistry of soil N emissions is traditionally thought to be dominated by N2O and N2. However, satellite, modelling, and laboratory studies illustrate that NOy emissions from soil are also important. Descriptions of soil emissions of NOy in climate models are underdeveloped, because details of the mechanisms leading to the formation of NOy species are lacking, of which, nitrous acid (HONO) is grossly lacking mechanistic detail. This represents a major gap in our understanding of a significant land-atmosphere interaction that prevents us from up-scaling these processes. There is a critical need to include these mechanisms into climate models since emissions of reactive N are expected to increase from estimates of 60 Tg-N y-1 to 80-130 Tg-N y-1 by 2100, coinciding with global temperature rises of between 2–4 °C. Soil emissions of HONO (estimated at 7.4–12.0 Tg-N y-1) play a crucial role in atmospheric chemistry, acting as a precursor for radicals that influence air quality and climate. However, the microbial sources and mechanisms responsible for HONO production in soils remain poorly understood. This project aims to elucidate the molecular mechanisms behind these emissions, focusing on the role of ammonia-oxidising archaea (AOA) and bacteria (AOB). Through controlled experiments with AOA and AOB cultures, soil incubations, and chemical assays, we will test the hypothesis that AOB actively release HONO while AOA do not, simultaneously determining the underlying molecular mechanisms. Additionally, this project will quantify the effects of HONO on soil organic matter turnover and composition. By serving as a radical source, HONO may stimulate the abiotic photooxidation of complex soil organic carbon pools. This could reveal a previously unknown link between the terrestrial nitrogen and carbon cycles, with implications for understanding and modelling soil carbon storage and climate change feedbacks. The research aligns with NERC priorities to advance predictive understanding of terrestrial biosphere responses and biogeochemical cycling feedbacks to environmental change. By elucidating a currently undefined source of NOy from soils, as well as its impacts on soil organic matter oxidation, the findings will reduce key uncertainties in Earth system models regarding soil N emissions and soil carbon storage. Improved representation of these processes in predictive models will benefit climate change forecasting and inform sustainable land management practices. This project leverages state-of-the-art techniques such as isotope labelling, microbial inhibition studies, spectroscopic analytical instrumentation, and genetic tools. These showcase UK expertise at the interface of environmental microbiology, soil biogeochemistry, and atmospheric chemistry, ultimately informing climate change mitigation strategies and sustainable land management practices. The core team is comprised of Ryan Mushinski (Project Lead) who has broad expertise in nitrogen cycle biogeochemistry, Gary Bending (Project Co-Lead), who has expertise in soil surface processes, and James Covington (Project Co-Lead) who has expertise in environmental engineering and sensor development/implementation. Specialised expertise will be facilitated by Marc Walker (Specialist in X-Ray Photoelectron Spectroscopy), Ben Breeze (Specialist in Raman Spectroscopy), Megan Purchase (Research and Innovation Associate, currently working on N-cycle projects with Mushinski), and a Senior Research Technician (to be hired) who will have expertise in traditional microbiology and -omics. This interdisciplinary collaboration will advance our understanding of connections between microbial processes, soil biogeochemistry, and atmospheric chemistry.
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